Read on table has exceeded the size warning threshold of…

Error message

Read on table %s has exceeded the size warning threshold of %,d bytes with ...

What it means

A coordinator read whose result exceeds the coordinator_read_size_warn_threshold_kb limit triggers this client warning (and a server log). The read still succeeds; it only flags that an unbounded partition/restricted query returned more data than expected.

Solutions

Add a LIMIT clause or narrow the partition key restrictions
Raise cassandra.coordinator_read_size_warn_threshold_kb if reads are legitimately large
Paging with driver fetch size to cap per-page memory
Investigate/fix large partitions with a compaction/data-model review

Example fix

// before
SELECT * FROM events WHERE device_id = ?; // partition grew to GBs
// after
SELECT * FROM events WHERE device_id = ? LIMIT 10000; // plus paging

Defensive patterns

Strategy: validation

Validate before calling

// enforce LIMIT/narrowed WHERE before issuing reads on wide tables
if (!cql.toLowerCase().contains("limit") && !hasPartitionKeyRestriction(cql)) {
    throw new IllegalArgumentException("unbounded read on wide table requires LIMIT");
}

Prevention

Always LIMIT unbounded reads
Enable paging with a bounded fetch size
Monitor coordinator_read_size metrics and large partitions

When it happens

Trigger: SELECT returning a result larger than cassandra.coordinator_read_size_warn_threshold_kb; raised in SelectStatement's result-processing path when result.shouldWarn(...) is true; threshold value -1 disables it.

Common situations: Large-partition tables read with wide SELECTs; missing LIMIT on queries; thresholds lowered after a memory incident; unbounded IN queries fanning out.

Understand the failure class

Background: payload too large / request exceeds maximum size: why libraries cap bytes and how to fix oversize payloads — this error's family across 50 libraries.

Related errors

AI-assisted analysis of apache/cassandra@88fd0f6a0e (2026-09-10). Data as JSON: /api/errors/66175f2b2ea90696. Report an issue: GitHub.

Appendix: source

Thrown at src/java/org/apache/cassandra/cql3/statements/SelectStatement.java:1139

            for (int i = 0; i < components.length; i++)
                result[i] = ByteBufferUtil.getArrayUnsafeNullable(components[i]);
            return result;
        }
        return new byte[][]{ ByteBufferUtil.getArrayUnsafeNullable(key) };
    }

    private void maybeWarn(ResultSetBuilder result, QueryOptions options)
    {
        if (!options.isReadThresholdsEnabled())
            return;
        ColumnFamilyStore store = cfs();
        if (store != null)
            store.metric.coordinatorReadSize.update(result.getSize());
        if (result.shouldWarn(options.getCoordinatorReadSizeWarnThresholdBytes()))
        {
            String msg = String.format("Read on table %s has exceeded the size warning threshold of %,d bytes", table, options.getCoordinatorReadSizeWarnThresholdBytes());
            ClientState state = ClientState.forInternalCalls();
            ClientWarn.instance.warn(msg + " with " + loggableTokens(options, state));
            logger.warn("{} with query {}", msg, asCQL(options, state));
            if (store != null)
                store.metric.coordinatorReadSizeWarnings.mark();
        }
    }

    private void maybeFail(ResultSetBuilder result, QueryOptions options)
    {
        if (!options.isReadThresholdsEnabled())
            return;
        if (result.shouldReject(options.getCoordinatorReadSizeAbortThresholdBytes()))
        {
            String msg = String.format("Read on table %s has exceeded the size failure threshold of %,d bytes", table, options.getCoordinatorReadSizeAbortThresholdBytes());
            ClientState state = ClientState.forInternalCalls();
            String clientMsg = msg + " with " + loggableTokens(options, state);
            ClientWarn.instance.warn(clientMsg);
            logger.warn("{} with query {}", msg, asCQL(options, state));
            ColumnFamilyStore store = cfs();

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